In the past, consumers were often beset by many problems associated with storage devices such as small capacity, frequent malfunction, low portability or poor circulation capability. Since recordable optical disks were available, and low-cost disk burners software and writers become widely popular, most of the problems mentioned above have been overcome. Nevertheless, to store digital video library data and powerful software requires huge amount of storage space. Even optical disks with storage capacity of 640-800 MB each set still cannot fully meet those requirements. There is a continuous demand for storing more data on every single optical disk.
Storage capacity is determined by recording density. With the recording density increased continuously, recording marks also are shrunk incessantly. However the size of optical spots on optical recording media is restricted by diffraction limits. Signals of recording marks with the sizes smaller than one half of optical spots cannot be detected or picked up. Hence optical spots cannot be shrunk unlimitedly. As a result, increasing of recording density also has limitation. Theoretically, in optical recording systems, the ultimate diffraction limitation for shrinking laser spots is about 0.6 ë/NA. Shorter laser wavelengths and converging lenses with higher numerical apertures (NA) can shrink laser spots much smaller to increase recording density. However blue light laser that has power over 30 mW and life time over 10,000 hours is expensive and not easy to produce. On the other hand, converging lenses of higher NA require very demanding optical and mechanical properties for the corresponding disks and disk drives. To overcome the bottleneck of diffraction limits, techniques such as Super-Resolution Near-Field Structure (Super-RENS) have been developed and introduced for adopting on various types of optical recording media, including Read-Only optical disks, Phase-Change and Magneto-optic optical disks.
Refer to FIG. 1 for the structure of a conventional optical disk. The optical disk is formed on a substrate 1 made from polycarbonate, and is sequentially covered by a recording layer 21 made from organic dyes such as cynanie dyes, azo dyes or phthalocyanines or other dyes that are sensitive to laser beams, a reflective layer 3 made of Au, Ag, Al, Cu or their alloys, and a protection layer 41 made from Ultra Violet (UV) curing resin. During burning or writing, burning light penetrates the substrate 1 and reaches the recording layer 21. Thermal energy of the penetrating light causes reaction in anthocyanosides contained in the recording layer 21 to perform recording function.
Refer to FIG. 2 for the structure of a conventional super-resolution optical disk. The near-field optical disk is formed on a substrate 1, and is sequentially covered by an under dielectric layer 51, a mask layer 52, an interface layer 53, a recording layer 21, an upper dielectric layer 54, and a protection layer 41. The recording layer 21 is made from a phase change material such as GeSbTe, or AgInSbTe; the under dielectric layer 51 and the interface layer 53 may be made from SiNx, SiO2, ZnS—SiO2; and the mask layer 52 may be made from Antimony (Sb), silver oxide (AgOx) or thermochromic organic compounds.
As conventional super-resolution optical disks require very a greater readout power, the recording marks on the recording layer and the optical disks are prone to damage. And life time of laser also suffers.